Process for preparing solvent-type acrylic pressure-sensitive adhesives and medical pressure-sensitive adhesive

ABSTRACT

The object is directed toward obtaining a process for preparing a solvent-type acrylic pressure-sensitive adhesive which is able to increase a conversion at polymerization in the preparation of the solvent-type acrylic pressure-sensitive adhesive to thereby solve the problems of odors associated with residual monomers and of attachment of gels on reactor walls, and further to provide improved productivity and workability. 
     In the preparation of the solvent-type acrylic pressure-sensitive adhesives, a monomer composition containing alkyl (meth)acrylate ester as its main component is subjected to solution polymerization under a closed condition at least in a latter stage of the polymerization reaction. In the polymerization, the solution to be polymerized is maintained at temperatures not lower than a boiling point thereof under normal pressures or at 50°-120° C.

FIELD OF THE INVENTION

The present invention relates to a method for preparing solvent-typeacrylic pressure-sensitive adhesives and a medical pressure-sensitiveadhesive utilizing the pressure-sensitive adhesives. More particularly,this invention relates to a method for preparing solvent-type acrylicpressure-sensitive adhesives for use in pressure-sensitiveadhesive-coated products in such forms as tapes and labels, and inmedical adhesive materials known as plasters and poultices; and to amedical pressure-sensitive adhesive utilizing the above solvent-typeacrylic pressure-sensitive adhesives for exhibiting a marked reductionin residual monomers content which may otherwise cause irritation tohuman body.

DESCRIPTION OF THE PRIOR ART

Conventionally, pressure-sensitive adhesive-coated products, such aspressure-sensitive adhesive tapes and sheets, have been widely used insuch applications as building materials, household electricalappliances, automotive cushioning materials, gap-filling tapes and thelike. Exemplary forms of such pressure-sensitive adhesive tapes andsheets include double-coated tapes having pressure-sensitive adhesivelayers provided on opposite surfaces of a substrate, foam tapes havingpressure-sensitive adhesive layers provided on at least one surface of afoam substrate, commonly-used pressure-sensitive adhesive tapes having apressure-sensitive adhesive layer provided on a single surface of asuitable substrate and the like.

In a medical field, medical adhesive materials have been widely employedwhich typically define a drug-containing pressure-sensitive adhesivelayer on a substrate. Another type of medical adhesive materialsincorporates a drug-free pressure-sensitive adhesive layer provided onat least one surface of a flexible sheet or tape substrate, such as foruse as adhesive bandages and the like.

Most representative of pressure-sensitive adhesives for use in varietiesof pressure-sensitive adhesive-coated products and medical adhesivematerials as mentioned above are solvent-type pressure-sensitiveadhesives comprised primarily of acrylic copolymers. This is because thesolvent-type acrylic pressure-sensitive adhesives exhibit excellentperformance characteristics including weatherability, durability, heatresistance, freeze resistance and water resistance.

The solvent-type acrylic pressure-sensitive adhesives have also comeinto widespread use as pressure-sensitive adhesives for constituting apressure-sensitive adhesive layer in medical adhesive materials, sincetheir physical properties, including a tack strength, can be readilycontrolled by combining various types of acrylic monomers.

The above-described, acrylic pressure-sensitive adhesives are obtainablegenerally by polymerizing a monomer composition containing as its maincomponent alkyl (meth)acrylate ester through solution or emulsionpolymerization, introducing additives and others to the resultingpolymer solution, and thereafter removing a solvent by drying. Whenmanufacturing medical adhesive materials, purposed drugs are added tothe above-mentioned polymer solution prior to removal of the solvent bydrying.

In medical practices, nitroglycerin-containing medical adhesivematerials of endermic absorption type are employed as remedies orprophylactics for heart disease such as angina pectoris, myocardialinfarction or heart failure. For example, Japanese Patent Laying-openNo. Sho 63-246325 discloses nitroglycerin-containing adhesive materialswhich utilize particular alkyl (meth)acrylate ester copolymers.

It is known that endermic preparations which utilize a medicalpressure-sensitive adhesive comprising a copolymer of alkyl(meth)acrylate ester and vinyl pyrrolidone, among the above-describedacrylic pressure-sensitive adhesives, are capable of exhibiting effectsof active ingredients in a short period after application as well astheir effectiveness for a prolonged period of time (Japanese Patentpublication No. Hei 3-70685).

BACKGROUND OF THE INVENTION

The widespread uses of the above-described, typical pressure-sensitiveadhesive-coated products have accompanied increased odor problemsassociated with residual acrylic monomers in the solvent-type acrylicpressure-sensitive adhesives. It is accordingly desired to providepressure-sensitive adhesive-coated products which release less odors asa result of reduced residual acrylic monomers. Furthermore, a strongdemand has arisen for the reduced odor of residual acrylic monomerswhich results in improved working atmospheres under whichpressure-sensitive adhesives are manufactured.

In the manufacture of the solvent-type acrylic pressure-sensitiveadhesives, various methods have been attempted to achieve odor reductionas mentioned above, examples of which include a method which attempts toenhance a conversion by increasing catalysts in amount, a method inwhich a polymerization reaction period is prolonged, a method whichattempts to add an increased amount of initiator in a latter stage ofthe polymerization reaction, and a method which involves the repeatedaddition of an initiator during the polymerization reaction.

However, those methods involving either adding the increased amount ofinitiator in the latter stage of the polymerization reaction orrepeatedly adding the initiator during the polymerization reactionsometimes caused significant change in pressure-sensitive adhesiveproperties thereof with time, while effective in lowering the residualmonomers content. In particular, among the pressure-sensitive adhesiveproperties, a holding power was observed to sometimes change largelywith time.

Also, in Japanese Patent Laying-open No. Sho 63-175086, a method isdisclosed which adds scavenger monomers after substantial completion ofpolymerization to reduce residual monomers. This method however resultedin substantially inadequate reaction of the scavenger monomers to leavethem as residues so that it failed to reduce odors to a satisfactoryextent.

While the reduced odor is pursued, an extremely reduced level ofresidual monomer concentration is highly sought for the medical adhesivematerials utilizing the solvent-type acrylic pressure-sensitiveadhesives, from additional considerations of preventing them fromcausing irritation, rash, itch and erythema to human body. To this end,a proposal has been made to provide a medical pressure-sensitiveadhesive which, prior to introducing drugs thereto, contains thereinresidual monomers in an amount not to exceed 0.2 weight % of a totalamount of the adhesive, on a solids content basis (Japanese PatentLaying-open No. Hei 5-131022).

A reduced degree of change in pressure-sensitive adhesion with time ishighly sought for the pressure-sensitive adhesive-coated products andmedical adhesive materials. In particular, for the medical adhesivematerials which are applied to human body in use, such a change inpressure-sensitive adhesion with time significantly affects comfort tohuman body during use thereof. For example, any increase inpressure-sensitive adhesion with time may cause damages to human skinwhen separating the medical adhesive material therefrom. In anotherevent where the pressure-sensitive adhesive layer increases itsstiffness with time, a stress thus applied to human skin during use maycause an increased degree of irritation to human skin.

On the other hand, any reduction in pressure-sensitive adhesion withtime may cause the medical adhesive material to be separated from humanskin or to be partially lifted during use thereof.

In recent years, it has been highly sought that the preparations ofendermic absorption type give reduced irritation to human skin duringuse thereof. One known method to reduce irritation to skin involvesadding to the pressure-sensitive adhesive a liquid component which iscompatible therewith. However, this method reduces cohesion of thepressure-sensitive adhesive to cause problems of staying or legginessthereof upon release from human skin, while effective in reducingirritation due to a plasticizing action of the liquid component.

In Japanese Patent Laying-open No. Hei 3-223212, a method is disclosedfor preventing reduction in cohesion of the pressure-sensitive adhesiveto relax or disperse a stress applied to human skin upon release of amedical adhesive material therefrom and thereby optimize a balancebetween adhesion and irritation to human skin, by introducing a liquidcomponent into a pressure-sensitive adhesive which is subsequentlycoated and followed by a crosslinking treatment to form oily gels. Therestill remains a problem that the residue of unreacted initiators in apressure-sensitive adhesive layer causes irritation to human skin.

Also, Japanese Patent Publication No. Hei 2-28978 discloses a methodwherein a pressure-sensitive adhesive is slightly pre-crosslinked withpolyfunctional monomers.

In the above case where a polymerization reaction period is prolonged toreduce residual monomers to a possible extent, a lowered productivityresults. In addition, the production of gels insoluble in a solvent isencouraged to possibly allow the gels to attach to reactor walls. As aresult, another problem arises which necessitates troubled operationssuch as for cleaning the reactor.

In particular, those methods, as described above, which eitherpre-crosslink the pressure-sensitive adhesive with polyfunctionalmonomers or effect polymerization utilizing a highly concentratedmonomer solution for increasing a molecular weight of a resultingcopolymer, accompany attachment of a large amount of gels onto thereactor walls. This consequently renders the operations, such as forcleaning the reactor, more troublesome to result in substantiallylowered productivity and workability in the manufacture of thepressure-sensitive adhesives.

Furthermore, the formation of the above-mentioned gels in apressure-sensitive adhesive solution causes reduced coatability of thepressure-sensitive adhesives and difficulty in obtaining preferredquality of endermic preparations.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a process for preparingsolvent-type acrylic pressure-sensitive adhesives whereby theabove-described problems encountered with the conventional solvent-typeacrylic pressure-sensitive adhesives can be solved and a conversion insolution polymerization can be enhanced to result in reduced odors ofthe pressure-sensitive adhesives commensurate with reduced amounts ofresidual monomers and improved productivity and workability.

It is also an object of the present invention to provide a highlyproductive and workable process for preparing solvent-type acrylicpressure-sensitive adhesives which, when employed as a medicalpressure-sensitive adhesive, exhibit less change in itspressure-sensitive adhesion with time.

It is further an object of the invention to provide a medicalpressure-sensitive adhesive which exhibits reduced odors associated withresidual monomers and less change with time in pressure-sensitiveadhesion, and which can be prepared with high productivity.

In accordance with a broad aspect of the invention, for the purposes ofsolving the problems as described above, a process for preparingsolvent-type acrylic pressure-sensitive adhesives is provided which ischaracterized by subjecting to solution polymerization a monomercomposition containing as its main component alkyl (meth)acrylate esterunder a closed condition at least in a latter stage of thepolymerization reaction in the presence of a radical initiator such thatthe residual initiator content in the pressure-sensitive adhesive is nothigher than 0.05 weight % of a total weight on a solids content basis.

In significance, the process for preparing a solvent-type acrylicpressure-sensitive adhesive in accordance with the present invention ischaracterized in that the monomer composition containing as its maincomponent alkyl (meth)acrylate ester is subjected to solutionpolymerization under a closed condition at least in the latter stage ofthe polymerization reaction.

Examples of solvents useful for the solution polymerization includeester solvents such as ethyl acetate, propyl acetate and butyl acetate;ketone solvents such as methyl ethyl ketone and cyclohexanone; aromaticsolvents such as benzene and toluene; and Cellosolve solvents such asmethyl Cellosolve and ethyl Cellosolve. The solvent may be employedalone or in combination with one or more other solvents.

In accordance with the present invention, the above-stated monomercomposition containing as its main component alkyl (meth)acrylate esteris subjected to solution polymerization under a closed condition atleast in the latter stage of the polymerization reaction. Such asolution polymerization under the closed condition may be effected byutilizing a polymerization reactor capable of defining a closed system.For example, a polymerization reactor is charged with a nitrogen gas todischarge air remaining in the reactor, followed by exclusion of thenitrogen gas using a vacuum pump to maintain an interior of the reactorunder vacuum pressure (10-200 mmHg) prior to effecting the solutionpolymerization.

Any reactor which has a pressure-resistant structure can be employed asthe above-described polymerization reactor. Various shapes of reactorscan be employed, examples of which include tank-, column-, andvessel-shaped reactors.

The solution polymerization needs to be effected under a closedcondition at least in the latter stage of the polymerization reaction.Accordingly, the solution polymerization can be effected under theclosed condition over entire steps of the polymerization reaction.Alternatively, such a closed condition can be applied at a stagesubsequent to a midpoint of the polymerization reaction.

The term "latter stage of the polymerization reaction", as used in theabove expression "under a closed condition at least in the latter stageof the polymerization reaction", means a state in which a conversion hasreached 95% or greater.

In accordance with the present process for preparing solvent-typeacrylic pressure-sensitive adhesives, the monomer composition containingas its main component alkyl (meth)acrylate ester, as hereinafterdescribed, is supplied into the above-described reactor. In such anevent, an undivided whole of the required amount of each monomercomponent which constitutes the monomer composition may be supplied atone time. Alternatively, suitably divided fractions thereof may besupplied at intervals. If necessary, a polymerization initiator ashereinafter described may be supplied to allow the polymerizationreaction to proceed. Likewise, an undivided whole or divided fractionsof a predetermined amount of the initiator may be supplied to thereactor at one time or at intervals.

In accordance with the present invention, the polymerization reaction iseffected under a closed condition, as described above. This allows areaction temperature to be set to not lower than a boiling point of asolvent used under normal pressures. In such an event that the reactiontemperature is set to not lower than the boiling point of the solventunder normal pressures, the equilibrium relationship of liquid and vaporphase portions is established within the reactor. On the other hand, inthe case of setting to not higher than the boiling point, a portion ofthe solvent is in a vapor state.

While a pressure developed within the reactor corresponds to a vaporpressure of the solvent at a predetermined temperature, the pressureincrease is too slight to create any substantial problems since gaseouscomponents are removed in an early stage.

As described above, effecting the polymerization at a temperature notlower than the boiling point of the solution to be polymerized undernormal pressures enables an enhanced conversion, as a result of whichthe residual monomers content can be reduced without using a prolongedreaction period.

In accordance with a particular aspect of the present invention, thesolution polymerization is effected under a closed condition at areaction temperature in the range of 50°-120° C. Effecting thepolymerization at an excessively low reaction temperature possiblyresults in a reduced reactivity, which necessitates a prolonged reactionperiod leading to a low productivity. On the other hand, a higherreaction temperature acts to increase an initial reactivity to possiblyresult in difficulty in controlling the polymerization reaction. Thus,the reaction temperature is preferably in the range of 50°-120° C., asstated above, more preferably in the range of 60°-100° C. When effectingthe reaction, the reaction temperature may be maintained constant orvaried within the above-specified preferred temperature range atsuitable time intervals.

THE MONOMER COMPOSITION

As described above, the present invention utilizes the monomercomposition containing as its main component alkyl (meth)acrylate ester.The type of alkyl (meth)acrylate ester is not particularly specified.Examples of alkyl residues contained in alkyl (meth)acrylate esterinclude methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, n-hexyl,isohexyl, 2-ethylhexyl, n-octyl, isooctyl, nonyl, decyl, lauryl, andstearyl groups. One or more of these groups may be used.

Any alkyl (meth)acrylate ester which contains one or more of theabove-listed alkyl residues can be employed in the present invention.Those alkyl (meth)acrylate esters can be employed which contain an alkylgroup having 1-18 carbon atoms. Illustrative of such alkyl(meth)acrylate esters include methyl (meth)acrylate, ethyl(meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl(meth)acrylate, isobutyl (meth)acrylate, n-hexyl (meth)acrylate,isohexyl (meth)acrylate, n-octyl (meth)acrylate, isobutyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate and anycombination of one or more of the above.

Those preferred for use as the above-mentioned alkyl (meth)acrylateester are alkyl (meth)acrylate esters containing an alkyl group having2-12 carbon atoms, examples of which include ethyl (meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate and lauryl (meth)acrylate.In the case where the number of carbon atoms contained in the alkylgroup is equal to 1 or higher than 12, the use of alkyl (meth)acrylateesters containing such an alkyl group possibly leads to inadequacy inpressure-sensitive adhesion of the solvent-type acrylicpressure-sensitive adhesives prepared.

As used herein, the term (meth)acrylate is intended to include acrylateand methacrylate.

The monomer composition in accordance with the present invention mayfurther incorporate a monomer copolymerizable with the alkyl(meth)acrylate ester. Such a monomer to be copolymered with the alkyl(meth)acrylate ester may be suitably selected from monomers which havebeen conventionally employed for copolymerization with alkyl(meth)acrylate esters in the preparation of acrylic pressure-sensitiveadhesives, depending upon the pursued pressure-sensitive adhesiveproperties. Illustrative of the monomers to be copolymerized with thealkyl (meth)acrylate ester are vinyl acetate, vinyl pyrrolidone,diacetone acrylamide, acrylonitrile, dimethyl acrylamide, ethyleneglycol mono(meth)acrylate ester, and styrene. These monomers can besuitably employed within a range which will not otherwise adverselyaffect pressure-sensitive adhesion and cohesion of the resultingpressure-sensitive adhesives. A maximum use amount of these monomers istypically 40 mole % of a total mole of the monomer composition.

In accordance with a particular aspect of the present invention, amonomer composition includes alkyl (meth)acrylate ester containing analkyl group having 2-12 carbon atoms, and a vinyl monomer(s)copolymerizable with alkyl (meth)acrylate ester. Examples of vinylmonomers include, but are not limited to, hydroxyl group-containingmonomers such as 2-hydroxyethyl (meth)acrylate, and 2-hydroxypropyl(meth)acrylate; carboxyl group-containing monomers such as (meth)acrylicacid, maleic acid, itaconic acid, and crotonic acid; and other monomersincluding methyl (meth)acrylate, vinyl acetate, styrene, fluoroacrylate,isononyl (meth)acrylate, (meth)acrylamide, acrylonitrile, and N-vinylpyrrolidone. Here, the use of the monomer composition comprising alkyl(meth)acrylate ester containing an alkyl group having 2-12 carbon atomsand the vinyl monomer(s) as described above allows adjustment ofphysical properties such as pressure-sensitive adhesion as well asimpartment of properties such as heat-resistance.

In accordance with another particular aspect of the present invention, amonomer composition containing as its main component alkyl(meth)acrylate ester incorporating an alkyl group having 6 or morecarbon atoms is employed wherein the alkyl (meth)acrylate ester includes40 weight % to 90 weight % of 2-ethylhexyl (meth)acrylate. In thisinstance, if the content of 2-ethylhexyl (meth)acrylate becomes larger,in an exemplary case where a medical adhesive material is preparedutilizing the resulting pressure-sensitive adhesives, it becomes morepossible that a pressure-sensitive adhesive layer thereof becomesstiffened and pressure-sensitive adhesion of the layer is reduced toallow the medical adhesive material to be easily separated. On the otherhand, in the event that the content of 2-ethylhexyl (meth)acrylate isexcessively small, the addition of highly concentrated drugs, such asnitroglycerin, to the monomer composition possibly causes reducedcohesion of the resulting pressure-sensitive adhesives so that thepressure-sensitive layer becomes softer to allow the adhesives to remainupon removal of the medical adhesive material. Thus, the monomercomposition is specified as containing 2-ethylhexyl (meth)acrylatepreferably in the range of 40-90 weight %, more preferably in the rangeof 60-80 weight %.

In the case where the number of carbon atoms contained in the alkyl(meth)acrylate ester is small, a saturated solubility therein of drugssuch as nitroglycerin is increased to result in enhanced affinity of theresulting copolymer for the drugs. This causes reduction of the drugproportion present in an adhesive layer portion facing toward human skinto result in reduced endermic absorption. Thus, the alkyl (meth)acrylateester which carries an alkyl group having 6 or more carbon atoms ispreferably employed, as described above.

Examples of the alkyl (meth)acrylate esters carrying an alkyl grouphaving 6 or more carbon atoms include n-hexyl (meth)acrylate, isohexyl(meth)acrylate, heptyl (meth)acrylate, n-octyl (meth)acrylate, isobutyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl(meth)acrylate, dodecyl (meth)acrylate, lauryl (meth)acrylate, andstearyl (meth)acrylate.

In addition to including the above-mentioned 2-ethylhexyl(meth)acrylate, the monomer composition further includes othermonomer(s) selected from one or more alkyl (meth)acrylate esters whichpreferably allow the resulting copolymer to exhibit a ball tack valuenot exceeding 2.

In accordance with another particular aspect of the present invention,the monomer composition includes 40-99 mole %, preferably 50-97 mole %of alkyl (meth)acrylate ester, and 1-60 mole %, preferably 3-50 mole %of vinyl pyrrolidone. In such a formulation, a suitable type of alkyl(meth)acrylate esters may be selectively employed depending upon itsratio to vinyl pyrrolidone in the resulting copolymer and the pursuedproperties.

The inclusion of alkyl (meth)acrylate ester is here specified as beingin the range of 40-99 mole % due to the following reasons. A lowerinclusion of alkyl (meth)acrylate ester possibly causes reducedpressure-sensitive adhesion of the resulting pressure-sensitiveadhesives whereas a higher inclusion thereof possibly causes reducedinitial drug release of a medical adhesive material prepared from theresulting pressure-sensitive adhesives.

In accordance with still another aspect of the present invention, themonomer composition includes alkyl (meth)acrylate ester, and apolyfunctional monomer which contains 2 or more polymerizable doublebonds per molecule, wherein the inclusion of the polyfunctional monomeris 0.001-0.1 moles per 100 moles of alkyl (meth)acrylate ester. In thisinstance, the alkyl (meth)acrylate ester employed may be selected fromalkyl (meth)acrylate esters which carry an alkyl group having 1-18carbon atoms, as described above.

Examples of the polyfunctional monomers include divinyl benzene,methylene bisacrylamide, ethylene glycol di(meth)acrylate ester,propylene glycol di(meth)acrylate ester, butylene glycoldi(meth)acrylate ester, hexylene glycol di(meth)acrylate ester,1,6-hexanediol di(meth)acrylate ester, polyethylene glycoldi(meth)acrylate ester, polypropylene glycol di(meth)acrylate ester, andtrimethylolpropane tri(meth)acrylate ester.

An unsatisfactory cohesive effect may result if the content of thepolyfunctional monomer is low. A higher content thereof possibly resultsin difficulty in adjusting a degree of polymerization during reaction,which facilitates gelling of a reaction solution, or makes it difficultto keep a copolymer solution in its stable state for a prolonged period,even though it is possible to obtain the copolymer solution withoutgelling thereof. Accordingly, the polyfunctional monomer is incorporatedin the range of 0.001-0.1 moles, preferably in the range of 0.003-0.07moles per 100 moles of the alkyl (meth)acrylate ester.

The addition of the polyfunctional monomer, as described above, allowsthe polymer to be partially crosslinked to be present in a "slightlycrosslinked" state, which enables production of a highly polymerizedcopolymer. This slight crosslink imparts appropriate cohesion toresulting pressure-sensitive adhesives and acts to prevent them fromremaining upon release thereof. Also, a pressure-sensitive adhesivesolution obtained exhibits an increased degree of stability.

In accordance with still another particular aspect of the presentinvention, the monomer composition includes 1-10 weight % of monomer(s)which has at least one reactive functional group selected from the groupconsisting of carboxyl, hydoxyl, amide, epoxy and amino groups. In thisinstance, the alkyl (meth)acrylate ester as employed may be selectedfrom alkyl (meth)acrylate esters which carry an alkyl group having 1-18carbon atoms, as described above. The type of such alkyl (meth)acrylateesters may be suitably selected depending upon a desired ratio thereofto a monomer(s) having the reactive functional group, as describedbelow, in the resulting copolymer, as well as the pursued properties.

Examples of the monomers having a carboxyl group include (meth)acrylicacid, crotonic acid, itaconic acid, fumaric acid, and maleic acid.

Examples of the monomers having a hydroxyl group include hydroxyethyl(meth)acrylate, hydroxypropyl (meth)acrylate, mono(hydoxyethyl) maleate,and propylene glycol mono(meth)acrylate.

Illustrative of the monomer having an amide group is (meth)acrylamide.

Illustrative of the monomer having an epoxy group is glycidyl(meth)acrylate.

Examples of the monomers having an amino group includedimethylaminoethyl (meth)acrylate and t-butylaminoethyl (meth)acrylate.

A lower content of the monomer having the reactive functional group, aslisted above, results in a reduced crosslinking effect thereof, ifcrosslinking is performed. A higher content thereof results in reducedpressure-sensitive adhesion of a copolymer obtained, and in poorapplicability thereof to human skin. Accordingly, the monomer having thereactive functional group is incorporated in the range of 1-10 weight %,preferably in the range of 2-8 weight % of a total weight of the monomercomposition.

More preferably employed as the monomer having the reactive functionalgroup is (meth)acrylic acid. The monomer composition contains preferably1-10 weight %, more preferably 2-8 weight % of (meth)acrylic acid. Thisis because if a content of (meth)acrylic acid becomes higher, when aformulation is made by adding a crosslinking agent to a resultingpressure-sensitive adhesive, a more stiffened pressure-sensitiveadhesive layer possibly results which has reduced pressure-sensitiveadhesion to allow easy separation thereof, and because a lower contentof (meth)acrylic acid possibly results in reduced crosslinking effect.

In accordance with still another particular aspect of the presentinvention, a solvent-type acrylic pressure-sensitive adhesive includesas its main component an acrylic copolymer having a number averagemolecular weight ranging from 10,000 to 500,000. The acrylic copolymerincludes an alkyl (meth)acrylate ester carrying an alkyl group having2-12 carbon atoms, and a vinyl monomer copolymerizable with the alkyl(meth)acrylate ester. If the number average molecular weight of theacrylic copolymer is less than 10,000, reduction in heat resistance of aresulting pressure-sensitive adhesive results. If it exceeds 500,000,excessive increase in viscosity of a resulting pressure-sensitiveadhesive results. Preferably, the acrylic copolymer has a number averagemolecular weight ranging from 100,000 to 500,000. In this case, amonomer composition for constituting the acrylic copolymer preferablyincludes 0.01-10 weight % of hydroxyl- and carboxyl-containing monomers.More preferably, the monomer composition includes 0.01-3 weight % ofhydroxyl-containing monomer and 1-10 weight % of carboxyl-containingmonomer. Inclusion of hydroxyl- and carboxyl-containing monomers inexcessively higher proportions causes reduced pressure-sensitiveadhesion of a resulting solvent-type acrylic pressure-sensitiveadhesive. Inclusion thereof in excessively lower proportions causesdecreased elastic modulus of a resulting solvent-type acrylicpressure-sensitive adhesive at elevated temperatures to result in itsreduced peel strength under constant load.

In the process for preparing a solvent-type acrylic pressure-sensitiveadhesive in accordance with the present invention, the above-describedpolymerization reaction may be carried out by further adding to theabove-mentioned monomers and solvent suitable additives which arecommonly used for polymerization reactions, e.g. other monomers orbelow-described polymerization initiators as required.

POLYMERIZATION INITIATOR

Conventional, thermally-activated radical initiators are useful as thepolymerization initiator optionally employed for the present invention,examples of which include organic peroxides such as peroxycarbonates,ketone peroxides, peroxyketals, hydroperoxides, dialkyl peroxides,diacyl peroxides, and peroxyesters (e.g. lauroyl peroxide, and benzoylperoxide); and azo compounds such as 2,2'-azobisisobutyronitrile,2,2'-azobis(2-methylbutyro-nitrile),2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(dimethylisobutyrate). The initiator for use may be suitably selected from thesepolymerization initiators, depending upon polymerization temperatures asused. Also, these polymerization initiators can be used alone or incombination.

The amount of the polymerization initiator used is preferably in therange of 0.0001-5 weight parts per 100 weight parts of the monomerswhich constitute the acrylic copolymer. However, in order to reduce theamount of residual monomers, it generally needs to be not lower than 0.1weight %.

THE RESIDUAL INITIATOR CONTENT

In accordance with a particular aspect of the present invention, asolution polymerization is carried out such that a residual initiatorcontent in the above-stated acrylic pressure-sensitive adhesive is nothigher than 0.05 weight % of a total amount, based on a solids contentbasis. As such, the residual initiator content of not higher than 0.05weight % of a total amount, on a solids content basis, is effective inpreventing undesirable change in adhesive properties of thepressure-sensitive adhesives with time.

The residual initiator present after the pressure-sensitive adhesive isprepared decomposes with time to generate radicals. This is consideredto cause the acrylic pressure-sensitive adhesive to undergo acrosslinking reaction through hydrogen abstraction, or to cause areaction of the radicals of adding to active groups present in drugs oradditives contained therein. It is accordingly desired that the residualinitiator content is as low as possible. The inventors of the presentapplication have discovered that any undesirable change of the adhesiveproperties with time can be effectively prohibited by allowing theresidual initiator content not to exceed 0.05 weight % of a total weightof the pressure-sensitive adhesive, on a solids content basis.

More preferably, the residual initiator content may be controlled within0.01 weight %, which range is more effective in preventing undesirablechange of the pressure-sensitive adhesive in adhesive properties withtime. If the residual initiator content exceeds 0.05 weight %, on asolids content basis, a significant change with time in adhesiveproperties possibly results.

An applicable method for reducing the residual initiator content in thepressure-sensitive adhesives is not particularly limited. One exemplarymethod involves elevating a temperature in the latter stage of thepolymerization reaction for maintaining a reaction solution at theelevated temperature for a time period not shorter than required.Considering a time period required for the polymerization, increasing atemperature in the latter stage of the polymerization is a preferredmethod to successfully reduce the residual initiator content in ashorter period of time. To illustrate this instance more specifically, amethod can be adopted in which the reaction solution is refluxed atboil, or maintained under pressure at a temperature not lower than aboiling point.

As described herein, a time period during which the reaction solution ismaintained at elevated temperatures means a time period until theresidual initiator content reaches 0.05 weight % or less of a totalweight of the reaction solution, on a solids content basis. Such a timeperiod can be calculated by substituting characteristic values(frequency factor, activation energy) of the initiator used, aconcentration of the initiator and a treatment temperature into thebelow-defined equations (1) and (2) for representing a reduction rate κof the initiator concentration.

     I!/ I!.sub.0 =e.sup.-κt                             ( 1)

    κ=Ae.sup.-Ea/RT                                      ( 2)

where, I! is a concentration of an initiator, I!₀ is an initial (t=0)concentration of the initiator, t is a time period, A is a frequencyfactor, Ea is an activation energy, R is a gas constant, and T is atemperature (absolute temperature).

THE MEDICAL PRESSURE-SENSITIVE ADHESIVE

In accordance with a broad aspect of the present invention, a medicalpressure-sensitive adhesive is provided which includes as its maincomponent a copolymer obtainable by subjecting a monomer compositioncontaining alkyl (meth)acrylate ester as its principal ingredient to asolution polymerization in the presence of an initiator that generatesradicals, and which contains a residual initiator in the range not toexceed 0.05 weight % of a total weight of the adhesive, on a solidscontent basis. Since the residual initiator content, in this instance,is in the range not to exceed 0.05 weight % of a total weight of theadhesive, on a solids content basis, as described above, any undesirablechange in adhesive properties of the medical pressure-sensitive adhesivewith time is effectively prevented. As also stated above, it is morepreferable that the content of the residual initiator is not higher than0.01 weight %.

Illustrative of the method for reducing the residual initiator contentto not higher than 0.05 weight % of a total weight, as stated above, isa method of copolymerizing the afore-described monomer composition undera closed condition at least in the latter stage of the polymerizationreaction.

Also, in accordance with a particular aspect of the present invention, amedical pressure-sensitive adhesive prepared in such a manner asdescribed above may be supported by a flexible backing to, as a whole,form an adhesive material in which the medical pressure-sensitiveadhesive is incorporated as a constituent. Such adhesive materials maybe used as adhesive bandages, medical dressing tapes and the like.

The above medical pressure-sensitive adhesive may further contain drugs.In such a case, the medical pressure-sensitive adhesive can beincorporated as a constituent in a medical adhesive material.Illustrative of the drugs which may be contained in thepressure-sensitive adhesive are, but not limited to, antiphlogisticanalgesic, antiphlogistic, coronary vasodilator, tranquilizers,antihypertensive, antibiotic, anesthetic, antibacterial materials,antihistamines, sex hormones, brain circulatory improvers andantiulcers. The drug content varies depending upon its type, but isgenerally in the range of 0.1-30 weight % of the pressure-sensitiveadhesive.

When necessary, an absorption accelerator, a solvent for the drug, atackifier, a crosslinking agent, a filler, or an antioxidant may besuitably incorporated in the pressure-sensitive adhesive layer.

Again, in the case of the medical pressure-sensitive adhesive whichfurther contains drugs, the medical pressure-sensitive adhesive ispreferably supported by a flexible backing, as described above.

Illustrative materials applicable to the flexible backing forconstituting the above-described adhesive materials include olefinderivatives such as polyethylene, polypropylene, polybutadiene,ethylene-vinyl acetate copolymer, ethylene-propylene copolymer,ethylene-alkyl (meth)acrylate copolymer, and polybutane; polystyrenessuch as styrene-isoprene-styrene copolymer, styrene-butadiene-styrenecopolymer, and hydrates thereof; vinylidene chloride derivatives such aspolyvinylidene chloride and vinylidene chloride-styrene copolymer; vinylchloride derivatives such as polyvinyl chloride, vinyl chloride-ethylenecopolymer, and vinyl chloride-alkyl acrylate ester copolymer; siliconeresins; polyesters such as polyethylene fluoride, polyethyleneterephthalate, and polybutylene terephthalate, polyurethanes, andpolyamides.

The flexible backing may also be of cotton or nonwoven fabrics laminatedon films prepared from the above resins. In addition, the flexiblebacking may take any form such as films, sheets, or tapes.

While any particular range of flexibility is not specified for theflexible backing, it is preferred that the flexible backing beconstructed to exhibit a sufficient flexibility to follow any motion ofa human body, since it is applied to a human skin in use. The thicknessof the flexible backing may vary depending upon the particular materialused. In the case of a film-form backing, the thickness thereof may benot higher than 500 μm, preferably 40-200 μm.

For the purpose of enhancing adhesion to the pressure-sensitive adhesivelayer, a surface of the film-form backing for supporting thepressure-sensitive adhesive layer may be optionally subjected to atreatment such as an undercoat processing, a corona discharge treatment,a chemical oxidization treatment, or an ozonization treatment.

The provision of the medical pressure-sensitive adhesive on the flexiblebacking results in the medical pressure-sensitive adhesive which takesthe form as the medical adhesive material. Generally, a release paper isapplied onto a surface of the pressure-sensitive adhesive layer of theadhesive material for shielding thereof from ambient atmosphere and forpreventing the pressure-sensitive adhesive from adhering to other partsprior to its purposed use.

Where the medical pressure-sensitive adhesive of the present inventionis incorporated as a constituent into the form of the adhesive material,it is preferable to place such a release paper on a surface of thepressure-sensitive adhesive. Those comprising polyethylene,polypropylene, or polyethylene terephthalate films can be employed asthe release paper. The thickness of the release paper is generally nothigher than 300 μm, preferably in the range of 10-200 μm.

In the preparation of the medical adhesive materials, conventional,general-purpose techniques can be suitably employed. Specifically,conventional techniques for coating pressure-sensitive adhesives, suchas bar coating and gravure coating can be employed. The thickness of thepressure-sensitive adhesive layer is not particularly limited, but isgenerally in the range of 20-1000 μm. The thickness not exceeding 20 μmmay be insufficient to incorporate a required amount of drugs in thepressure-sensitive adhesive layer and to provide a satisfactorypressure-sensitive adhesion. The thickness exceeding 1000 μm possiblycauses poor diffusion of the drugs contained in the pressure-sensitiveadhesive portion present in the vicinity of the backing so that theutilization of the drugs is reduced.

DESCRIPTION OF THE PREFERRED EXAMPLES EXAMPLE 1

A closable, polymerization reactor having a pressure-resistant structureand equipped with a stirrer, temperature controller, nitrogen line,heating and cooling jackets was employed. The reactor was first purgedwith nitrogen gas to discharge air remaining in the reactor andmaintained at vacuum pressure (about 60 mmHg). Supplied by suction intothe reactor were 1000 g of ethyl acrylate, 800 g of octyl acrylate, 200g of methyl methacrylate and 2000 g of ethyl acetate, each previouslybubbled with nitrogen gas. While the mixture was stirred at a rate of 30rpm and an interior of the reactor was maintained at 80° C., a 0.1weight % solution of lauroyl peroxide in ethyl acetate was added theretobatchwise, i.e. 10 times at intervals of 2 hours to effectpolymerization. An exothermic heat of the polymerization was controlledby reducing a jacket temperature (temperature of a heat transfer medium)so that the interior of the reactor was maintained at 80° C. Thepolymerization was continued for 24 hours. After cooling, ethyl acetatewas introduced and mixed so that a polymer concentration was thinned to30 weight %. A pressure-sensitive adhesive solution thus obtained waswithdrawn from the reactor.

EXAMPLE 2

Polymerization was carried out in the same manner as in Example 1 exceptthat the reaction temperature for polymerization was changed from 80° C.to 60° C.

EXAMPLE 3

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the modifications thatlauroyl peroxide was replaced by benzoyl peroxide and the reactiontemperature was changed from 80 ° C. to 110° C.

EXAMPLE 4

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the followingmodifications: A mixture of 1600 g of 2-ethylhexyl methacrylate, 200 gof 2-ethylhexyl acrylate, 200 g of dodecyl methacrylate and 2000 g ofethyl acetate as a solvent was used as the monomer composition. Thereaction temperature was changed to 60° C.

EXAMPLE 5

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 4 except that the reactiontemperature was changed from 60° C. to 80° C.

EXAMPLE 6

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 4 with the modifications thatlauroyl peroxide was replaced by benzoyl peroxide and the reactiontemperature was changed to 110° C.

EXAMPLE 7

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 4 with the followingmodifications: A mixture of 800 g of 2-ethylhexyl methacrylate, 500 g of2-ethylhexyl acrylate, 700 g of dodecyl methacrylate and 2000 g of ethylacetate as a solvent was used as the monomer composition. The reactiontemperature was changed to 80° C.

EXAMPLE 8

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 4 with the followingmodifications: A mixture of 1800 g of 2-ethylhexyl methacrylate, 200 gof 2-ethylhexyl acrylate and 2000 g of ethyl acetate as a solvent wasused as the monomer composition. The reaction temperature was changed to80° C.

COMPARATIVE EXAMPLE 1

A reflux condenser was mounted to one of the inlet lines of thepolymerization reactor employed in Example 1 to open an upper portionthereof. The monomer composition and the solvent of Example 1 wereintroduced into the reactor. The interior atmosphere of the reactor wasthen substituted by nitrogen. The mixture was then heat refluxed whilestirred at 30 rpm and charged with 30 ml/min of nitrogen. The jackettemperature was maintained at a temperature 3° C. higher than theinterior temperature of the reactor. The 0.1 weight % solution oflauroyl peroxide in ethyl acetate was added in the same manner as inExample 1 to effect polymerization. Since the interior temperature ofthe reactor decreases as the polymerization proceeds, a control wasapplied to maintain the jacket temperature 3° C. above the interiortemperature of the reactor. The polymerization was continued for 24hours. After cooling, ethyl acetate was introduced and mixed so that apolymer concentration was thinned to 30 weight %. A pressure-sensitiveadhesive solution was thus obtained.

COMPARATIVE EXAMPLE 2

A reflux condenser was mounted to one of the inlet lines of thepolymerization reactor employed in Example 4 to open an upper portionthereof. The monomer composition of Example 1 was introduced into thereactor the interior atmosphere of which was then substituted bynitrogen. The mixture was then heat refluxed while stirred at 30 rpm andcharged with 30 ml/min of nitrogen. The polymerization was performedinitially at 90° C. and in about 10 hours at 80° C. while the 0.1 weight% solution of lauroyl peroxide in ethyl acetate, as a polymerizationinitiator, was added batchwise 10 times at time intervals of 2 hours.Since the interior temperature of the reactor decreases as thepolymerization proceeds, controlling was applied to maintain the jackettemperature (temperature of heat transfer medium) at a temperature 3° C.above the interior temperature of the reactor. The polymerization wascontinued for 24 hours. After cooling, ethyl acetate was introduced andmixed so that a polymer concentration was thinned to 30 weight %. Apressure-sensitive adhesive solution was thus obtained.

COMPARATIVE EXAMPLE 3

A pressure-sensitive adhesive solution was prepared in the same manneras employed in Comparative Example 2 except that the monomer compositionof Example 7 was used as a monomer composition.

COMPARATIVE EXAMPLE 4

A pressure-sensitive adhesive solution was prepared in the same manneras employed in Comparative Example 2 except that the monomer compositionof Example 8 was used as a monomer composition.

A list of the formulations, solvents, polymerization initiators, andreaction temperatures employed respectively in Examples 1-8 andComparative Examples 1-4, as described above, is provided in thefollowing Tables 1 and 2. In Table 1 and 2, "EA" represents ethylacrylate, "OA"represents octyl acrylate, "MMA" represents methylmethacrylate, "EHM" represents 2-ethylhexyl methacrylate, "EHA"represents 2-ethylhexyl acrylate, "DM" represents dodecyl methacrylate,and "EtAc" represents ethyl acetate. "LPO" and "BPO" as polymerizationinitiators represent lauroyl peroxide and benzoyl peroxide,respectively.

                  TABLE 1    ______________________________________    ALKYL    (METH)ACRYLATE    ESTERS    (g)                EtAc              TEMP.    EA         OA      MMA     (g)   INITIATOR                                             (°C.)    ______________________________________    Example 1            1000   800     200   2000  LPO     80    Example 2            1000   800     200   2000  LPO     60    Example 3            1000   800     200   2000  BPO     110    Comparative            1000   800     200   2000  LPO     --    Example 1    ______________________________________

                  TABLE 2    ______________________________________    ALKYL    (METH)ACRYLATE    ESTERS    (g)                EtAc              TEMP.    EHM        EHA     DM      (g)   INITIATOR                                             (°C.)    ______________________________________    Example 4            1600   200     200   2000  LPO     60    Example 5            1600   200     200   2000  LPO     80    Example 6            1600   200     200   2000  BPO     110    Example 7             800   500     700   2000  LPO     80    Example 8            1800   200     --    2000  LPO     80    Comparative            1600   200     200   2000  LPO     --    Example 2    Comparative             800   500     700   2000  LPO     --    Example 3    Comparative            1800   200     --    2000  LPO     --    Example 4    ______________________________________

EXAMPLES 9-18

A pressure-sensitive adhesive solution was obtained in the same manneras in Example 1 with the following modifications: Supplied into thereactor was a mixture of alkyl acrylate ester, vinyl pyrrolidone andethyl acetate respectively in their predetermined amounts as listed inthe following Table 3. The polymerization initiators and reactiontemperatures as shown in the following Table 3 were utilized.

COMPARATIVE EXAMPLES 5-10

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Comparative Example 1 with thefollowing modifications: A mixture of alkyl acrylate ester, vinylpyrrolidone and ethyl acetate respectively in their predeterminedamounts as listed in the following Table 3 was utilized.

In the following Table 3, the terms commensurate with those used inTables 1 and 2 are intended to provide the same meanings. "VP"represents vinyl pyrrolidone. In each box on the VP column, the valuesgiven in top and bottom rows indicate a weight (g) and a proportion(mole %) of VP in the monomer composition, respectively.

                  TABLE 3    ______________________________________    ALKYL    (METH)ACRYLATE    ESTERS          VP    (g)             (g)     EtAc           TEMP.    EHA      OA      EA     (mol %)                                  (g)  INITIATOR                                               (°C.)    ______________________________________    Ex. 9 1500   --      --   500   2000 LPO     60                              35.6    Ex. 10          1500   --      --   500   2000 LPO     80                              35.6    Ex. 11          1500   --      --   500   2000 BPO     110                              35.6    Ex. 12          1900   --      --   100   2000 LPO     80                               8.0    Ex. 13          1100   --      --   900   2000 LPO     80                              57.6    Ex. 14          --     800     1000 200   2000 LPO     60                              11.2    Ex. 15          --     800     1000 200   2000 LPO     80                              11.2    Ex. 16          --     800     1000 200   2000 BPO     110                              11.2    Ex. 17          --     900     1000 100   2000 LPO     80                               5.7    Ex. 18          --     700      400 900   2000 LPO     80                              50.9    Comp. 1500   --      --   500   2000 LPO     --    Ex. 5                     35.6    Comp. 1900   --      --   100   2000 LPO     --    Ex. 6                      8.0    Comp. 1100   --      --   900   2000 LPO     --    Ex. 7                     57.6    Comp. --     800     1000 200   2000 LPO     --    Ex. 8                     11.2    Comp. --     900     1000 100   2000 LPO     --    Ex. 9                      5.7    Comp. --     700      400 900   2000 LPO     --    Ex. 10                    50.9    ______________________________________

EXAMPLES 19-23

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the followingmodifications: Alkyl (meth)acrylate esters as shown in the followingTable 4, polyfunctional monomers and ethyl acetate were utilized intheir respective proportions as listed in the following Table 4. Thepolymerization initiators and the reaction temperatures were selected asshown in the following Table 4.

COMPARATIVE EXAMPLES 11-13

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Comparative Example 1 except that themonomer compositions contained alkyl (meth)acrylate esters andpolyfunctional monomers in their respective proportions as listed in thefollowing Table 4.

EXAMPLES 24-29

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the followingmodifications: Compositions containing alkyl (meth)acrylate esters shownin the following Table 5, vinyl pyrrolidone (VP), polyfunctionalmonomers and ethyl acetate were utilized in their respective proportionsas given in the following Table 5. The polymerization initiators and thereaction temperatures were selected as listed in the following Table 5.

COMPARATIVE EXAMPLES 14 AND 15

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Comparative Example 1 except that themonomer compositions contained alkyl (meth)acrylate esters shown in thefollowing Table 5, vinyl pyrrolidone and polyfunctional monomers intheir respective proportions as listed in the following Table 5.

EXAMPLES 30-32

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the followingmodifications: Compositions containing alkyl (meth)acrylate esters shownin the following Table 6, polyfunctional monomers and ethyl acetate wereutilized in their respective proportions as given in the following Table6. The polymerization initiators and the reaction temperatures wereselected as listed in the following Table 6.

COMPARATIVE EXAMPLE 16

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Comparative Example 1 except that themonomer compositions contained alkyl (meth)acrylate esters shown in thefollowing Table 6 and polyfunctional monomers in their respectiveproportions as listed in the following Table 6.

In Tables 4, 5 and 6, if the terms used therein for representingrespective monomers correspond to those used in Tables 1-3, they areintended to provide the same meanings. "BA" represents butyl acrylate,"BM" represents butyl methacrylate, "HDA" represents 1,6-hexanedioldiacrylate ester, and "PGDM" represents polyethylene glycoldimethacrylate ester.

                                      TABLE 4    __________________________________________________________________________    ALKYL    (METH)ACRYLATE POLYFUNC.                          ETHYL    ESTERS         MONOMER                          ACETATE                                INITIATOR                                      TEMP.    (mol)          (mol)  (g)   (g)   (°C.)    __________________________________________________________________________    Ex. 19         BA   --   HDA    2000  LPO   80         10        0.0008    Ex. 20         BA   --   HDA    2000  LPO   60         10        0.0008    Ex. 21         BA   --   HDA    2000  BPO   110         10        0.0008    Ex. 22         BA   BM   PGDM   2000  LPO   80          6   4    0.0005    Ex. 23         BA   EA   HDA    2000  LPO   80           8.5              1.5  0.004    Comp.         BA   --   HDA    2000  LPO   --    Ex. 11         10        0.0008    Comp.         BA   BM   PGDM   2000  LPO   --    Ex. 12          6   4    0.0005    Comp.         BA   EA   HDA    2000  LPO   --    Ex. 13           8.5              1.5  0.004    __________________________________________________________________________

                                      TABLE 5    __________________________________________________________________________    ALKYL    (METH)ACRYLATE    ESTERS           POLYFUNC.    (g)           VP MONOMER    EHA    OA  EA (g)    (g) EtAc      TEMP.    (mol)  (mol)               (mol)                  (mol)                     TYPE                         (mol)                             (g) INITIATOR                                       (°C.)    __________________________________________________________________________    Ex. 24        1500           --  -- 500                     HDA 0.227                             2000                                 LPO   80        8.14      4.50   0.001    Ex. 25        1500           --  -- 500                     PGDM                         0.107                             2000                                 LPO   80        8.14      4.50   0.0005    Ex. 26        1500           --  -- 500                     HDA 0.045                             2000                                 BPO   110        8.14      4.50   0.0002    Ex. 27        -- 800 1000                  200                     HDA 0.227                             2000                                 LPO   80           4.34               9.99                  1.80   0.001    Ex. 28        -- 800 1000                  200                     PGDM                         0.107                             2000                                 LPO   80           4.34               9.99                  1.80   0.0005    Ex. 29        -- 800 1000                  200                     HDA 0.045                             2000                                 BPO   110           4.34               9.99                  1.80   0.0002    Comp.        1500           --  -- 500                     HDA 0.227                             2000                                 LPO   --    Ex. 14        8.14      4.50   0.001                             HDA    Comp.        -- 800 1000                  200                     HDA 0.227                             2000                                 LPO   --    Ex. 15 4.34               9.999                  1.80   0.001    __________________________________________________________________________

                                      TABLE 6    __________________________________________________________________________    ALKYL    (METH)ACRYLATE    ESTERS         POLYFUNC.    (g)            MONOMER    EHM     EHA DM      (g) EtAc      TEMP.    (mol)   (mol)                (mol)                   TYPE (mol)                            (g) INITIATOR                                      (°C.)    __________________________________________________________________________    Ex. 30         1600            200 200                   HDA  0.113                            2000                                LPO   80         8.08            1.09                0.79    0.0005    Ex. 31         1400            300 300                   PGDM 0.214                            2000                                LPO   80         7.07            1.63                1.18    0.001    Ex. 32         1600            200 200                   HDA  0.045                            2000                                BPO   110         8.08            1.09                0.79    0.0002    Comp.         1600            200 200                   HDA  0.113                            2000                                LPO   --    Ex. 16         8.08            1.09                0.79    0.0005    __________________________________________________________________________

EXAMPLES 33-39

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the followingmodifications: The monomer compositions were utilized which containedalkyl (meth)acrylate esters shown in the following Table 7, monomershaving a polyfunctional group and ethyl acetate as the solvent in theirrespective proportions as given in the following Table 7. Thepolymerization initiators as shown in the following Table 7 wereutilized as the polymerization initiator. The reaction temperature wasset to values as listed in the following Table 7.

COMPARATIVE EXAMPLES 17-19

A pressure-sensitive adhesive solution was obtained in ccordance withthe polymerization procedures of Comparative Example 1 with thefollowing modifications: The alkyl (meth)acrylate esters shown in thefollowing Table 7, monomers having a polyfunctional group and ethylacetate were used in their respective proportions as given in thefollowing Table 7. The polymerization initiators as shown in thefollowing Table 7 were utilized, and the reaction temperature was set totemperatures as given in the following Table 7.

In the following Table 7, if the terms used therein for representingrespective monomers and polymerization initiators correspond to thoseused in Tables 1-6, they are intended to provide the same meanings."AAc" represents acrylic acid, "HEMA" represents hydroxylethylmethacrylate, "AAm" represents acrylamide, "GMA" represents glycidylmethacrylate, and "DAEA" represents dimethylaminoethyl acrylate.

                                      TABLE 7    __________________________________________________________________________    ALKYL         MONOMER    (METH)ACRYLATE                  HAVING REAC.                          ETHYL    ESTERS        FUNC. GP.                          ACETATE                                INITIATOR                                      TEMP.    (g)           (g)     (g)   (g)   (°C.)    __________________________________________________________________________    Ex. 33        EHA  1900 AAc 100 2000  LPO   60    Ex. 34        EHA  1900 AAc 100 2000  LPO   80    Ex. 35        EHA  1900 AAc 100 2000  BPO   110    Ex. 36        BA   1900 HEMA                      100 2000  LPO   80    Ex. 37        OA   1950 AAm  50 2000  LPO   80    Ex. 38        EHA  1900 GMA 100 2000  LPO   80    Ex. 39        OA   1900 DAEA                      100 2000  LPO   80    Comp.        EHA  1900 AAc 100 2000  LPO   --    Ex. 17    Comp.        OA   1900 HEMA                      100 2000  LPO   --    Ex. 18    Comp.        BA   1950 AAm  50 2000  LPO   --    Ex. 19    __________________________________________________________________________

EXAMPLES 40-44

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Example 1 with the followingmodifications: The monomer compositions were utilized which containedalkyl (meth)acrylate esters shown in the following Table 8, acrylicacid, and ethyl acetate in their respective proportions as given in thefollowing Table 8. Those shown in the following Table 7 were utilized asthe polymerization initiator. The reaction temperature was set to valuesas listed in the following Table 8.

COMPARATIVE EXAMPLES 20-22

A pressure-sensitive adhesive solution was obtained in accordance withthe polymerization procedures of Comparative Example 1 with thefollowing modifications: The monomer compositions were utilized whichcontained alkyl (meth)acrylate esters shown in the following Table 8,acrylic acid, and ethyl acetate in their respective proportions as givenin the following Table 8. Lauryl peroxide was used as the polymerizationinitiator.

In Table 8, if the terms used therein correspond to those used in Tables1-7, they are intended to provide the same meanings.

                  TABLE 8    ______________________________________    ALKYL    (METH)ACRYLATE    ESTERS          AAc     EtAc   INITIATOR                                           TEMP.    (g)             (g)     (g)    (g)     (°C.)    ______________________________________    Ex. 40          EHA      1900     100   2000 LPO     60    Ex. 41          EHA      1900     100   2000 LPO     80    Ex. 42          EHA      1900     100   2000 BPO     110    Ex. 43          EHM      1950      50   2000 LPO     80    Ex. 44          OA       1800     200   2000 LPO     80    Comp. EHA      1900     100   2000 LPO     --    Ex. 20    Comp. EHM      1950      50   2000 LPO     --    Ex. 21    Comp. OA       1800     200   2000 LPO     --    Ex. 22    ______________________________________

The pressure-sensitive adhesive solutions obtained in the above Examples1-44 and Comparative Examples 1-22 were withdrawn to thereafter observewhether or not gels deposited on the reactor walls. Furthermore, for thepurpose of dissolving the gels deposited on the reactor walls, if theywere present, ethyl acetate was introduced into the polymerizationreactor, heated for 3 hours and refluxed for cleaning. After withdrawalof ethyl acetate, observation was made to see the remaining geldeposits.

The results showed that no deposition of gels on the reactor walls wasnoticed at a point when the withdrawal of ethyl acetate was completed,throughout Examples 1-44. In contrast, throughout Comparative Examples1-22, the attachment of gels on the reactor walls was noticed at a pointwhere the withdrawal of ethyl acetate was completed, and those attachedgels were noticed in all those examples to remain even after cleaningwas applied in the manner as described above.

EXAMPLES 45-50

A closable, polymerization reactor having a pressure-resistant structureand equipped with a stirrer, temperature controller, nitrogen line,heating and cooling jackets was employed. Monomers and a solvent(s) asspecified in Table 9 were first introduced into an reactor maintained inits opened condition, stirred at 30 rpm while purged with nitrogenbefore a temperature thereof was elevated. The solution to bepolymerized was then heat fluxed for about 30 minutes at a boiling pointthereof to remove an excess of oxygen. The solution was thereaftercooled to an initial polymerization temperature. After an interior ofthe reactor was placed under a closed condition, benzoyl peroxide as apolymerization initiator was added to initiate polymerization. Theaddition of benzoyl peroxide was carried out batchwise in 4 stages suchthat 10% of its whole amount was added at an initial stage of thepolymerization, 20% in an hour, 20% in 2 hours, and 50% in 3 hours fromthe initial stage. Table 10 shows (1) reaction temperatures, (2)pressures and (3) boiling points of solutions when the given reactionperiods elapsed. The final conversions were measured by a gaschromatography. Those values in % are given in Table 10. The molecularweights of acrylic copolymers obtained are also given in Table 10.

COMPARATIVE EXAMPLES 23-28

Polymerization was carried out in the same manner as employed inExamples 45-50 except that the interior of the reactor was placed underan opened condition after cooled to initial polymerization temperaturesas specified in Table 10. Table 11 shows (1) reaction temperatures, (2)pressures and (3) boiling points of solutions when the given reactionperiods elapsed; final conversions; and molecular weights of acryliccopolymers obtained.

In Tables 9-11, "EA" represents ethyl acrylate, "BA" represents butylacrylate, "2EHA" represents 2-ethylhexyl 5 acrylate, "AAc" representsacrylic acid, and "EtAc" represents ethyl acetate, as acrylic copolymerconstituent monomers.

                  TABLE 9    ______________________________________            Example           Comparative Example            45-47 48, 49  50      23-25 26, 27                                              28    ______________________________________    MONOMERS    EA        0       20      0     0     20    0    BA        80      50      50    80    50    50    2EHA      17      25      48    17    25    48    AAc       3       5       2     3     5     2    SOLVENTS    EtAc      100     80      60    100   80    60    TOLUENE   0       20      40    0     20    40    INITIATOR 0.05    0.20    0.5   0.05  0.20  0.5    ______________________________________     *IN UNIT OF PARTS BY WEIGHT

                                      TABLE 10    __________________________________________________________________________    IN THE LAPSE OF HOURS    (1) REAC. TEMP. (2) PRESS. (3) B.P. OF SOLN.    Ex.       ELAPSED                        CONV.                                          MOL.    No.       TIME 0    2    4    6    8  (HR.)                                      (%) WT.    __________________________________________________________________________    45 (1)  93   85   90   88   85 (°C.)                                      99.3                                          0.2       (2)  1    →                      2    →                                   (atm)  mill.       (3)  93   85   83   81   80 (°C.)    46 (1)  98   95   88   86   85 (°C.)                                      99.7                                          0.2       (2)  2    →                      →                           →                                   (atm)  mill.       (3)  93   85   83   81   80 (°C.)    47 (1)  80   80   90   90   90 (°C.)                                      99.5                                          0.2       (2)  0.3               →                 0.5                    →                      2  →                           2.5                              →                                3  (atm)  mill.       (3)  93   85   83   81   80 (°C.)    48 (1)  80   80   80   95   93 (°C.)                                      99.7                                           0.25       (2)  0.2               →                 0.5                    →                      0.8                         →                           2  →                                2  (atm)  mill.       (3)  98   92   86   83   82 (°C.)    49 (1)  105  100  97   95   93 (°C.)                                      99.9                                           0.25       (2)  2    →                      →                           →                                   (atm)  mill.       (3)  98   92   86   83   82 (°C.)    50 (1)  100  100  100  100  100                                   (°C.)                                      99.8                                          0.1       (2)  0.5               →                 0.8                    →                      1  →                           1.5                              →                                2  (atm)  mill.       (3)  108  104  100  95   90 (°C.)    __________________________________________________________________________

                                      TABLE 11    __________________________________________________________________________        IN THE LAPSE OF HOURS    Comp.        (1) REAC. TEMP. (2) PRESS. (3) B.P. OF SOLN.    Ex. ELAPSED                       CONV.                                          MOL.    No. TIME 0    2    4    6    8 (HR.)                                      (%) WT.    __________________________________________________________________________    23  (1)  93   85   83   81   80                                   (°C.)                                      97.0                                          0.2        (2)  1    →                       →                            →                                   (atm)  mill.        (3)  93   85   83   81   80                                   (°C.)    24  (1)  80   80   80   80   80                                   (°C.)                                      93.2                                          0.2        (2)  1    →                       →                            →                                   (atm)  mill.        (3)  93   85   83   81   80                                   (°C.)    25  (1)  80   80   80   79   78                                   (°C.)                                      95.5                                          0.2        (2)  0.3                →                  0.5                     →                       1    →                                   (atm)  mill.        (3)  93   85   83   81   80                                   (°C.)    26  (1)  80   80   80   80   80                                   (°C.)                                      93.7                                           0.25        (2)  0.2                →                  0.5                     →                       0.8                          →                            0.9                               →                                 1 (atm)  mill.        (3)  98   92   86   83   82                                   (°C.)    27  (1)  98   92   86   83   82                                   (°C.)                                      98.3                                           0.25        (2)  1    →                       →                            →                                   (atm)  mill.        (3)  98   92   86   83   82                                   (°C.)    28  (1)  108  104  100  95   90                                   (°C.)                                      98.7                                          0.1        (2)  1    →                       →                            →                                   (atm)  mill.        (3)  108  104  100  95   90                                   (°C.)    __________________________________________________________________________

EXAMPLES 51-59

A five-necked flask equipped with a stirrer, thermometer, refluxcondenser, nitrogen line and dropping funnel was employed. Into theflask is charged predetermined amounts of monomers and ethyl acetate asshown in Table 12. An interior of the flask was then substituted bynitrogen. The solution was stirred at 30 rpm while charged with nitrogenbefore its temperature was elevated. Thereafter, the solution wasrefluxed for about 30 minutes at a boiling point thereof to discharge anexcess of oxygen, followed by cooling to polymerization temperatures asgiven in Table 13.

1.0 g of lauroyl peroxide was dissolved into ethyl acetate until a wholeamount thereof reaches 30 ml to prepare an initiator solution.Polymerization was carried out by dropping the initiator solution insuch patterns as shown in the following Table 13 into the monomersolutions maintained at their respective polymerization temperatures.After the polymerization was completed, ethyl acetate was introducedinto a system for mixing therewith so that a solids concentrationreached 30 weight %. The pressure-sensitive adhesive solutions were thusprepared.

COMPARATIVE EXAMPLES 29-34

Polymerization was carried out utilizing the equipments used in Examples51-59 and in the same manner as in Examples 51-59. However, thepredetermined amounts of monomers and ethyl acetate as given in thefollowing Table 12 were introduced, and such polymerization conditionsas indicated in Table 13 were utilized. After the polymerization wascompleted, ethyl acetate was introduced into a system for mixingtherewith so that a solids concentration reached 30 weight %, as alsoperformed in Examples 51-59. The pressure-sensitive adhesive solutionswere thus prepared.

EXAMPLES 60-68

A closed, polymerization reactor having a pressure-resistant structureand equipped with a stirrer, temperature controller, nitrogen line,reflux condenser, heating and cooling jackets was employed.

The nitrogen gas was purged into the reactor to discharge air remainingin the reactor, and subsequently removed by a vacuum pump to maintainthe interior of the reactor under a degree of vacuum at about 60 mmHg.The predetermined amounts of monomers and ethyl acetate, previouslybubbled with nitrogen gas, were supplied by suction into the reactor intheir respective proportions as given in the following Table 14.

The monomer solution was then stirred at 30 rpm while the interior ofthe reactor was maintained at polymerization temperatures specified inthe following Table 15.

The reactor was maintained under a closed condition in Examples 60, 61,63, 64, 66 and 67, and a top of the reflux condenser was remained openin Examples 62, 65 and 68. In Examples 51-68, the following procedureswere taken into practice.

An initiator solution was prepared in the same manner as employed inExample 51. Polymerization was carried out by dropping the initiatorsolution in such patterns as shown in the following Table 15 into themonomer solutions maintained at their respective polymerizationtemperatures. However, in Examples 62, 65 and 68, the polymerizationreactor was brought into a closed condition after the final introductionof the initiator solution was made, before the polymerization wasperformed.

Again, in Examples 60-68, after the polymerization was completed, ethylacetate was introduced into a system for mixing therewith so that asolids content therein reached 30 weight %, as performed in Examples51-59. The pressure-sensitive adhesive solutions were thus prepared.

COMPARATIVE EXAMPLES 35-40

Utilizing the equipments as employed in Examples 60-68, polymerizationwas carried out in the nitrogen stream under the conditions as specifiedin the following Table 15, wherein the monomers and ethyl acetate weresupplied in the predetermined amounts as given in the following Table 14and the top of the reflux condenser was maintained opened from thebeginning till the end.

After the polymerization was completed, ethyl acetate was introduced andmixed so that a solids concentration therein reached 30 weight %. Thepressure-sensitive adhesive solutions were thus prepared.

The terms as used in Table 12 and Table 13 for indicating monomers havethe following meanings:

EHA . . . 2-ethylhexyl acrylate, EA . . . ethyl acrylate, OA . . . octylacrylate, VP . . . N-vinyl pyrrolidone, EHMA . . . 2-ethylhexylmethacrylate, DM . . . dodecyl methacrylate.

                  TABLE 12    ______________________________________                               ETHYL    MONOMERS (g)               ACETATE    EHA         EA    OA     VP  EHMA   DM   (g)    ______________________________________    Example 51            75                 25              100    Example 52            75                 25              150    Example 53            75                 25              100    Example 54      50    40   10              100    Example 55      50    40   10              150    Example 56      50    40   10              150    Example 57            10                     78     12    50    Example 58            10                     78     12    75    Example 59            10                     78     12    50    Comparative            75                 25              100    Example 29    Comparative            75                 25              100    Example 30    Comparative     50    40   10              150    Example 31    Comparative     50    40   10              150    Example 32    Comparative            10                     78     12    50    Example 33    Comparative            10                     78     12    50    Example 34    ______________________________________

                                      TABLE 13    __________________________________________________________________________    POL.    AMT. OF INITIATOR ADDED AT THE LAPST                                       POL.    TEMP.   OF HOURS DURING POLYMERIZATION (ml)                                       TIME    (°C.)            0 H               2 H 4 H                      6 H 8 H                             10 H                                 15 H                                    20 H                                       (h)    __________________________________________________________________________    Ex. 51        75*)            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       25    Ex. 52        70*)            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       26    Ex. 53        60*)            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       30    Ex. 54        75  0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       30    Ex. 55        70*)            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       30    Ex. 56        60*)            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       26    Ex. 57        75*)            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       25    Ex. 58        70*)            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       26    Ex. 59        60*)            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       30    Comp.        75  0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       23    Ex. 29    Comp.        60  0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       25    Ex. 30    Comp.        75  0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       25    Ex. 31    Comp.        60*)            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       23    Ex. 32    Comp.        75  0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       23    Ex. 33    Comp.        60  0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       25    Ex. 34    __________________________________________________________________________

                  TABLE 14    ______________________________________                               ETHYL    MONOMERS (g)               ACETATE    EHA        EA     OA     VP   EHMA  DM   (g)    ______________________________________    Example 60            75                 25              100    Example 61            75                 25              150    Example 62            75                 25              100    Example 63     50     40   10              100    Example 64     50     40   10              150    Example 65     50     40   10              150    Example 66            10                      78    12    50    Example 67            10                      78    12    75    Example 68            10                      78    12    50    Comparative            75                 25              100    Example 35    Comparative            75                 25              100    Example 36    Comparative    50     40   10              150    Example 37    Comparative    50     40   10              150    Example 38    Comparative            10                      78    12    50    Example 39    Comparative            10                      78    12    50    Example 40    ______________________________________

                                      TABLE 15    __________________________________________________________________________    POL.    AMT. OF INITIATOR ADDED AT THE LAPST                                       POL.    TEMP.   OF HOURS DURING POLYMERIZATION (ml)                                       TIME    (°C.)            0 H               2 H 4 H                      6 H 8 H                             10 H                                 15 H                                    20 H                                       (h)    __________________________________________________________________________    Ex. 60         70/100            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       21.0    Ex. 61        70/90            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       21.5    Ex. 62         60/110            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       21.0    Ex. 63        75/85            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       22.0    Ex. 64        70/90            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       21.5    Ex. 65         60/100            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       20.5    Ex. 66         70/100            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       21.0    Ex. 67        70/90            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       21.5    Ex. 68         60/110            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       21.0    Comp.        70  0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       28.0    Ex. 35    Comp.        60  0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       30.0    Ex. 36    Comp.        70/77            0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       24.0    Ex. 37    Comp.        60/77            0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       25.0    Ex. 38    Comp.        70  0.3               0.3 0.3                      0.6 1.5                             3.0 9.0                                    15.0                                       28.0    Ex. 39    Comp.        60  0.5               0.5 1.0                      1.0 2.0                             5.0 5.0                                    15.0                                       30.0    Ex. 40    __________________________________________________________________________

In Table 13, the polymerization temperatures to which an asterisk * isgiven mean that they were elevated after the lapse of 20 hours from theinitiation of polymerization for refluxing at boil (liquidtemperature=77° C.).

In Table 15, the polymerization temperature is given, throughoutExamples 60-68, as (temperature maintained within 20 hours from theinitiation of polymerization)/(temperature maintained after the lapse of20 hours from the initiation of polymerization). For example, 70/100, asgiven in Example 60, means that the temperature was maintained at 70° C.within 20 hours from the initiation of polymerization and that thetemperature was maintained at 100° C. after the lapse of 20 hours fromthe initiation of polymerization.

Also in Comparative Examples 37 and 38, the solution was refluxed atboil (liquid temperature=77° C.) after the lapse of 20 hours from theinitiation of polymerization.

EVALUATION OF EXAMPLES AND COMPARATIVE EXAMPLES

The concentration of the polymerization initiator remaining in theresulting pressure-sensitive adhesives was determined by one of methodsfor analyzing active oxygens in peroxides, i.e. an iodine titrationmethod (titration with a 1/10N sodium thiosulfate solution), using aniodine compound (potassium iodide). The determined concentrations inweight % relative to a total solids content in the pressure-sensitiveadhesive are given in Table 16 and Table 17.

(2) HOLDING POWER EVALUATION OF THE PRESSURE SENSITIVE ADHESIVE AFTERINCORPORATED INTO A MEDICAL ADHESIVE MATERIAL

For the pressure-sensitive adhesive solutions obtained in Examples 51-55and 60-64, and Comparative Examples 29-32 and 35-38, eachpressure-sensitive adhesive solution was knife coated onto a siliconesurface of a polyethylene terephthalate film and thereafter dried toform a coating having a thickness of 50 μm. The coating was applied to asubstrate comprising a laminated film of polyethylene terephthalate andethylene-vinyl acetate copolymer to form a laminate.

For the pressure-sensitive adhesive solutions obtained in Examples 56and 65, mixed with each pressure-sensitive solution was isosorbidenitrate in its concentration of 10 weight % relative to a total solidscontent. The mixture was knife coated onto a silicone surface of apolyethylene terephthalate film and thereafter dried to form a coatinghaving a thickness of 50 μm. The coating was applied to a substratecomprising a laminated film of polyethylene terephthalate andethylene-vinyl acetate copolymer to form a laminate.

For the pressure-sensitive adhesive solutions obtained in Examples 57-59and 60-68, and Comparative Examples 33, 34, 39 and 40, mixed with eachpressure-sensitive solution was nitroglycerin in its concentration of 10weight % relative to a total solids content. The mixture was knifecoated onto a silicone surface of a polyethylene terephthalate film andthereafter dried to form a coating having a thickness of 50 μm. Thecoating was applied to a substrate comprising a laminated film ofpolyethylene terephthalate and ethylene-vinyl acetate copolymer to forma laminate.

In either cases, the drying of the coated pressure-sensitive adhesiveswas effected at 60° C. for 30 minutes to prepare the medical adhesivematerials. Also, a portion of each medical adhesive material obtainedwas cut apart to provide a pressure-sensitive tape which was then sealedwithin an aluminum package for placement in a constant temperature bathmaintained at 60° C. for 2 weeks (acceleration test).

In evaluating the change of adhesive properties with time, the holdingpower was measured in the following procedures for each medical adhesivematerial left 3 days after the pressure-sensitive adhesive was coated,and for each medical adhesive material subjected to the aboveacceleration test.

Holding power measurement . . . A strip of 25 mm wide and 150 mm longwas prepared. The strip was adhered to a stainless steel plate so as tobe in contact therewith in an area of 25×25 mm. The unadhered portion ofthe strip was folded toward inside, that is, a pressure-sensitiveadhesive layer was folded onto itself. A pressure was applied forbonding thereof by one reciprocal movement of a 2 kg roller over thetest piece at a speed of 300 mm/min. A sample comprising the above testpiece bonded to a stainless steel plate was held in a constanttemperature bath maintained at 40° C. for 20 minutes or longer. One endof the test piece was then secured by a fastener so that the test piecewas suspended vertically. A 1 kg weight was attached to an end portionof the unadhered, folded part, and a time period (holding time period)until the weight dropped was measured. The results are given in thefollowing Table 16 and Table 17.

                  TABLE 16    ______________________________________    RESIDUAL    INITIATOR     HOLDING TIME (MIN.)    (wt. %)       AFTER COATING                               AFTER ACC. TEST    ______________________________________    Example 51            0.037     52           55    Example 52            0.021     39           40    Example 53            0.000     65           65    Example 54            0.009     224          226    Example 55            0.000     175          176    Example 56            0.029     38           39    Example 57            0.045     25           28    Example 58            0.028     18           19    Example 59            0.000     29           28    Comparative            0.154     58           114    Example 29    Comparative            0.364     72           298    Example 30    Comparative            0.067     236          354    Example 31    Comparative            0.143     294          579    Example 32    Comparative            0.208     27           48    Example 33    Comparative            0.459     35           109    Example 34    ______________________________________

                  TABLE 17    ______________________________________    RESIDUAL    INITIATOR     HOLDING TIME (MIN.)    (wt. %)       AFTER COATING                               AFTER ACC. TEST    ______________________________________    Example 60            0.001     58           60    Example 61            0.016     41           47    Example 62            0.000     74           72    Example 63            0.038     241          259    Example 64            0.022     182          189    Example 65            0.037     32           34    Example 66            0.002     23           24    Example 67            0.023     19           21    Example 68            0.000     27           28    Comparative            0.101     52           107    Example 35    Comparative            0.348     78           263    Example 36    Comparative            0.073     227          341    Example 37    Comparative            0.052     212          324    Example 38    Comparative            0.135     29           41    Example 39    Comparative            0.447     38           112    Example 40    ______________________________________

For the pressure-sensitive adhesives of Examples 51-68, the residualinitiator contents thereof are all within 0.05 weight %, as apparentfrom Table 16 and Table 17. It is accordingly understood that the aboveholding time period shows little change between after coating and afterthe acceleration test.

In contrast, the residual initiator contents are high, i.e. not lowerthan 0.05 weight % in Comparative Examples 29-40. In probableassociation with this, the strips subjected to the acceleration test arefound to show a marked increase in holding power relative to those aftercoating. It is accordingly considered that the use of the medicaladhesive materials of Comparative Examples 29-40 for application to askin surface possibly causes significant discomfort during use sincetheir holding powers are markedly increased as stated above.

In Comparative Example 32 wherein the temperature was increased in thelatter stage of the polymerization to reflux at boil, the reduction ofthe residual initiator content was not sufficient, probably due to theshortened treatment period, so that the holding power after subjected tothe acceleration test was significantly increased. Similarly, inComparative Examples 37 and 38 wherein refluxing at the highest reactiontemperature, i.e. at boil was effected in the latter stage of thepolymerization using conventional open-system facilities, the residualinitiator contents were only reduced to 0.073 weight % and 0.052 weight%, respectively, as apparent from Table 17. For this reason, the ratherincreased holding powers resulted after the acceleration test relativeto those after coating, as apparent from Table 17.

EFFECTS OF THE INVENTION

As hereinbefore described, in accordance with the process of the presentinvention for preparing a solvent-type pressure-sensitive adhesive, themonomer composition containing alkyl (meth)acrylate ester as its maincomponent is subjected to solution polymerization under a closedcondition at least in the latter stage of the polymerization, whichenables prevention of the solvent from being refluxed during thepolymerization reaction to result in the largely reduced deposition ofgels on the reactor walls. This significantly improves workability inthe step of cleaning the reactor after the preparation of thepressure-sensitive adhesive was completed, and enables the increasedproductivity of the solvent-type acrylic adhesive.

Also, since the solvent-type acrylic adhesives thus obtained retain nogels, they can be suitably employed as medical pressure-sensitiveadhesives. In addition, the formulations suitable for use in a medicaladhesive material can be prepared utilizing the solvent-type acrylicadhesives.

Also, in the present invention, since the solution polymerization iscarried out under a closed condition at least in the latter stage of thepolymerization reaction, the polymerization reaction can be effected attemperatures not lower than a boiling point of the solvent at normalpressures. This enables an enhanced conversion to result in an increasedproductivity as well as a reduced residual monomers content whereby odorproblems associated with the residual monomers can be solved.

In the process of the present invention for preparing a solvent-typeacrylic adhesive, the solution polymerization is carried out under aclosed condition at reaction temperatures in the range of 50°-120° C.This eliminates the necessity of a prolonged polymerization to result inan increased productivity, and also facilitates controlling thepolymerization reaction.

Also, in the process of the present invention for preparing asolvent-type acrylic adhesive, when the solution polymerization iscarried out such that a residual initiator content in thepressure-sensitive adhesive is not higher than 0.05 weight % of a totalweight, based on a solids content basis, the low, residual initiatorcontent is effective in preventing the pressure-sensitive adhesive fromchanging with time in adhesive properties, particularly in holdingpower. Accordingly, in the case that the medical pressure-sensitiveadhesive is constructed using such a solvent-type acrylic adhesive, thediscomfort given to users can be reduced since the adhesive propertiesof the pressure-sensitive adhesive layer hardly change with time. Thatis, any stress or irritation to skin can be reduced. Also, since itspressure-sensitive adhesion is hard to change with time, the separationor partial lifting of the medical adhesive materials from skin duringuse thereof can be prevented.

Also, the solvent-type acrylic pressure-sensitive adhesives having theresidual initiator content within 0.05 weight % of a total weightthereof, on a solids content basis, can be obtained reliably andefficiently in a short period by subjecting the monomer compositioncontaining alkyl (meth)acrylate ester as its main ingredient to solutionpolymerization in the presence of the initiator which generates radicalsand under a closed condition at least in the latter stage of thepolymerization reaction.

Furthermore, in the process of the present invention for preparing asolvent-type acrylic pressure-sensitive adhesive, alkyl (meth)acrylateester and vinyl monomers copolymerizable with alkyl (meth)acrylate esterare utilized as the monomer composition containing alkyl (meth)acrylateester as its main ingredient when preparing the solvent-type acrylicpressure-sensitive adhesive including as its main component the acryliccopolymer having a number average molecular weight of 10,000-500,000. Inthis instance, if the solution polymerization is carried out under aclosed condition at least in the latter stage of the polymerizationreaction, the reaction temperatures can be set to not lower than theboiling point of the monomer solution under normal pressures, so thatthe conversion can be increased and odors associated with the presenceof residual monomers can be reduced.

We claim:
 1. A medical pressure-sensitive adhesive which has as its maincomponent a copolymer prepared by subjecting a monomer compositioncontaining alkyl (meth) acrylate ester as its main ingredient tosolution polymerization under vacuum in the presence of an initiatorwhich generates radicals, said medical pressure-sensitive adhesivehaving a residual initiator content not higher than 0.05 weight % of atotal weight, on a solid content basis.
 2. The medicalpressure-sensitive adhesive as set forth in claim 1 wherein saidcopolymer is polymerized under a closed condition at least in the latterstage of the polymerization so that the residual initiator content isnot higher than 0.05 weight % of a total weight on a solids contentbasis.
 3. The medical pressure-sensitive adhesive as set forth in claim1 or 2 characterized in that said medical pressure-sensitive adhesive issupported on a flexible backing to take a form of an adhesive material.4. The medical pressure-sensitive adhesive as set forth in claim 1 or 2characterized by further containing drugs.
 5. The medicalpressure-sensitive adhesive as set forth in claim 4 characterized inthat said medical pressure-sensitive adhesive is supported on a flexiblebacking to take a form of an adhesive material.
 6. A process forpreparing an acrylic pressure-sensitive adhesive wherein a monomercomposition containing alkyl (meth)acrylate ester as its main componentis subjected to solution polymerization under a closed condition atleast in a latter stage of the polymerization reaction in the presenceof a radical initiator such that the residual initiator content in thepressure-sensitive adhesive is not higher than 0.05 weight % of thetotal weight on a solids content basis.
 7. The process for preparing anacrylic pressure-sensitive adhesive as set forth in claim 1, whereinsaid monomer composition is subjected to solution polymerization atreaction temperatures not lower than the boiling point of the solutionto be polymerized.
 8. The process for preparing an acrylicpressure-sensitive adhesive as set forth in claim 1, whereinpolymerization is effected at reaction temperatures in the range of50°-120° C. when said solution polymerization under the closed conditionis carried out.
 9. The process for preparing an acrylicpressure-sensitive adhesive as set forth in any one of claims 1-3,wherein said monomer composition includes alkyl (meth)acrylate estercarrying an alkyl group having 2-12 carbon atoms and vinyl monomerscopolymerizable with the alkyl (meth)acrylate ester.
 10. The process forpreparing an acrylic pressure-sensitive adhesive as set forth in any oneof claims 1-3, wherein said monomer composition is alkyl (meth)acrylateester carrying an alkyl group having 6 or more carbon atoms whichcontains 40-90 weight % of 2-ethylhexyl methacrylate.
 11. The processfor preparing an acrylic pressure-sensitive adhesive as set forth in anyof claims 1-3, wherein said monomer composition includes 40-99 mole % ofalkyl (meth)acrylate ester and 1-60 mole % of vinyl pyrrolidone.
 12. Theprocess for preparing an acrylic pressure-sensitive adhesive as setforth in claim 4, wherein said monomer composition is a compositionwhich includes alkyl (meth)acrylate ester and a polyfunctional monomerhaving 2 or more polymerizable double bonds per molecule and contains0.01-0.1 moles of the polyfunctional monomer per 100 moles of said alkyl(meth)acrylate ester.
 13. The process for preparing an acrylicpressure-sensitive adhesive as set forth in any one of claims 1-3,wherein said monomer composition contains alkyl (meth)acrylate ester and1-10 weight % of a monomer having at least one functional group selectedfrom the group consisting of carboxyl, hydroxyl, amide, epoxy and aminogroups.
 14. The process for preparing an acrylic pressure-sensitiveadhesive as set forth in claim 13, wherein said monomer having at leastone functional group selected from the group consisting of carboxyl,hydroxyl, amide, epoxy and amino groups is (meth)acrylic acid.
 15. Themedical pressure-sensitive adhesive as set forth in claim 2, wherein aconversion is higher than 95% in said latter stage of thepolymerization.
 16. The medical pressure-sensitive adhesive as set forthin claim 1, wherein the polymerization is performed under vacuumpressure of from 10 to 200 mmHg.
 17. The process for preparing anacrylic pressure-sensitive adhesive as set forth in claim 6, wherein aconversion is higher than 95% in said latter stage of thepolymerization.
 18. The process for preparing an acrylicpressure-sensitive adhesive as set forth in claim 6, wherein thepolymerization is performed under vacuum pressure of from 10 to 200mmHg.
 19. The process for preparing an acrylic pressure-sensitiveadhesive as set forth in claim 6, wherein the polymerization is effectedat reaction temperatures in the range of 60° to 100° C.